Re: The 'cold' function attribute and GDB

Simon Marchi
2019-05-02T18:08:06Z

On 2019-05-02 12:45 p.m., Eli Zaretskii wrote:
>> Date: Thu, 2 May 2019 08:59:19 -0700
>> From: Kevin Buettner <kevinb@redhat.com>
>> Cc: gdb-patches@sourceware.org, simark@simark.ca
>>
>>> This is with GCC 8.2.0, btw.
>>
>> I'll give this a try.
>>
>> I have GCC 8.3.1 and 9.0.1 readily available. Do you think it's
>> important to use 8.2.0 ?

I just built gcc from git, and built emacs with it, and I get pretty much the
same ranges as you:

<1><66a91b>: Abbrev Number: 141 (DW_TAG_subprogram)
<66a91d> DW_AT_name : (indirect string, offset: 0xf3fe3): print_vectorlike
<66a921> DW_AT_decl_file : 1
<66a922> DW_AT_decl_line : 1365
<66a924> DW_AT_decl_column : 1
<66a925> DW_AT_prototyped : 1
<66a925> DW_AT_type : <0x65a0ff>
<66a929> DW_AT_ranges : 0x234cf0
<66a92d> DW_AT_frame_base : 1 byte block: 9c (DW_OP_call_frame_cfa)
<66a92f> DW_AT_GNU_all_call_sites: 1
<66a92f> DW_AT_sibling : <0x66cf15>

And the ranges:

00234cf0 000000000057da80 000000000057e88f
00234cf0 00000000004173b1 00000000004173b6
00234cf0 <End of list>

In my case, the second range, the cold one, is one instruction long, just the call to emacs_abort:

00000000004173b1 <print_vectorlike.cold>:
4173b1: e8 77 ce ff ff callq 41422d <emacs_abort>

Can you give the steps to reproduce the bug that leads us there?

In the mean time, I tried to do "start", followed by "set $pc = 0x4173b1", and this is what I get as my
first frame:

#0 0x00000000004173b1 in print_vectorlike (obj=0x1, printcharfun=0x7fffffffdd18, escapeflag=<optimized out>, buf=0xa0 <error: Cannot access memory at address 0xa0>) at /home/smarchi/src/emacs/src/print.c:1824

The parameter values are bogus, and the rest of the frames are corrupted, because I don't have
the stack I would normally have when executing this code. But we can see that the full symbol
was found: the arguments are printed, and the function name is correct (doesn't include .cold).

So it looks like some debugging of this problem on Windows will be needed :(

Simon